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- [Narrator] We know that COVID-19 spreads
through close contact,
making crowded areas, potentially deadly.
But when some members of a crowd are immune,
that begins to change.
This is called herd immunity.
It's what's protecting much of the population
from diseases like measles and polio.
And it's what's driving vaccine efforts today.
- The notion is,
there'll be enough people in that community who're immune,
so that when you go to pass the disease on,
you won't be able to pass it on
in a way that it will propagate.
- [Narrator] Economies around the world
can then safely reopen.
And some say it could put an end to the pandemic.
But the path to herd immunity includes big hurdles.
And experts say it could take years or more
if we get there at all.
We'll explain.
When scientists want to determine
how fast a virus might spread,
they look at its reproduction number.
This represents how many people could catch the virus
from a single infected person
when no one in the population is immune.
- That helps us to understand how much additional immunity
or other measures do we need to implement in place
to start to bring this up epidemic under control,
as opposed to having it expand over time.
- [Narrator] For example, for the polio virus,
one infected person will pass the disease
to five to seven people on average.
And from there, the disease will spread exponentially.
For the novel coronavirus,
scientists say an infected person will likely spread it
to two to four other people.
- That number can go down
as more people become immune or also in response
to measures like wearing masks, social distancing.
- [Narrator] When enough people
in a population gain immunity,
the chances of infecting others goes down.
And once it falls below a certain point,
it becomes harder and harder for the virus to spread.
As a result,
the majority of the community becomes protected,
not just those who are immune.
- Over time, the virus dissipates
and really essentially goes away
or circulates at very low levels in that community.
- [Narrator] Scientists say there are two ways
a population can reach herd immunity.
Through broad infection or more commonly vaccination.
To get there through infection,
the majority of the population needs to get sick
from a virus and develop molecular defenses
like antibodies that fight off the disease.
But this method would be deadly.
Experts estimate that less than 5%
of people worldwide have had COVID-19.
We get scientists say, to reach herd immunity,
that number would have to be between 60 and 70%.
- If we need to get to 60 or 70% infections,
that's obviously more
than 4 billion people getting infected.
And that would be tens of millions of people dying,
it would be hundreds of millions
of people getting very, very sick, it would be devastating.
It would be devastating for the entire global population,
would be devastating for the economies
and it is wholly unnecessary.
- [Narrator] There's also no guarantee
that broad infection would lead to eradication.
- We don't have many examples
of complete immunity to viruses through natural infection
because people continue to get born in a population.
So every year you have a new set of babies who are born
and they're going to be susceptible to that infection.
- [Narrator] Experts say the more efficient way
to herd immunity, is through vaccination.
That's how scientists eradicated smallpox.
- Because of vaccines,
childhood mortality has dropped dramatically.
Kids no longer die of measles.
They don't die of a lot of the diseases
that kids used to die of.
Obviously smallpox was one of the horrible scourges
of humanity throughout the entire human history.
We don't talk about smallpox anymore.
Because our vaccination campaign ultimately led
to the eradication of smallpox.
- [Narrator] But experts say they don't know enough
about the coronavirus to predict the likelihood
of reaching herd immunity.
And that there are large barriers to getting there.
Even with a vaccine.
One challenge is that scientists don't yet know
how long natural immunity might last.
- There's a big difference between being immune
for six months and being immune for 10 years.
- [Narrator] For other respiratory viruses, like the flu,
antibodies tend to decrease over time.
Scientists have observed this
in some patients recovering from COVID-19.
But there isn't enough data yet
to be certain about what that means for immunity.
How long a COVID-19 vaccine will last is still unclear,
but it may be measured in months or years,
rather than a lifetime.
Another challenge,
not all vaccines in development will work the same.
And their effectiveness could differ.
- The better a vaccine is
in terms of the level of protection
and the duration of protection,
the further it gets you towards that point of herd immunity.
But even a less than perfect vaccine,
if given to enough people in the population,
can help reduce the spread.
- [Narrator] Scientists are working quickly
to test dozens of vaccines already in development.
But it could take years
to figure out which ones are more effective.
So even if people get vaccinated,
some could be more susceptible to catching the virus.
But even as cases surge around the world,
some experts are optimistic.
- We suspect that people
who have gotten COVID-19 remain protected
for many months, at least
because we haven't seen a large number of repeat infections.
We also know that it's possible
to give people multiple vaccinations.
So I'm hopeful that we can develop over time a vaccine that
even if we do need to give multiple doses,
will be highly effective
in generating the levels of immunity that we need
to beat this disease.
- [Narrator] If this happens,
global herd immunity may be possible.
But until then, health experts have stressed
that measures like social distancing
and wearing masks will be key to slowing down the spread.
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